The purification of liquid has become of great interest in many industries. In particular, pure water is used for many industrial purposes rather than merely as drinking water. For example, pure water is used in processes for producing semiconductor chips, in power plants, in the petrochemical industry and for many other purposes. Ion exchange resins, reverse osmosis filtration and electrodialysis techniques have been used to reduce the concentration of ions in a liquid.
Electrodeionization apparatus have recently been used with more frequency to reduce the concentration of ions in a liquid. The term "electrodeionization" generally refers to an apparatus and process for purifying liquids which combine ion exchange resins, ion exchange membranes and electricity to purify the liquids. An electrodeionization module comprises alternating arrangements of cation permeable membranes and anion permeable membranes defining compartments therebetween. In alternating compartments, there is provided ion exchange resin. Those compartments are known as diluting compartments.
The compartments which generally do not contain ion exchange resin are known as the concentrating compartments. Ions migrate from the diluting compartments through ion exchange resin and ion permeable membranes into the concentrating compartments by the introduction of current. The liquid flowing through the concentrating compartments is discarded or partially recycled and the purified liquid flowing through the diluting compartments is recovered as deionized liquid product.
U.S. Pat. No. 4,636,296 which issued Jan. 13, 1987 to Kunz discloses an apparatus and method for the demineralization of aqueous solutions. An aqueous liquid is passed through alternating separate layers of cation exchange resin and anion exchange resin. This approach is cumbersome, electrode intensive and some distortion of the layers will likely occur during service.
U.S. Pat. No. 5,308,467 which issued May 3, 1994 to Sugo et al. discloses an electrically regenerable demineralizing apparatus which has a demineralizing compartment. Ion exchange groups are disposed on monofilaments, woven fabric of monofilaments or nonwoven fabric of monofilaments by radiation-initiated graft polymerization. This ion exchange material is contained within the demineralizing compartment.
The use of such monofilaments in demineralizing apparatus is expensive and, therefore, has not been readily accepted by purchasers of liquid purification apparatus.
It is desirable to have an arrangement of ion exchange material in the diluting compartments of electrodeionization apparatus which does not use monofilaments and which allows various types of ion exchange material to be arranged in the diluting compartment in non-layer arrangement but still allowing the liquid to be purified to come into contact with discrete zones of two types of ion exchange material.